The present invention pertains generally to the field of controls for gain drying bins. Grain drying bins are widely used in the farming industry to reduce the moisture content of grain to acceptable levels prior to storage or marketing. Often the grain is initially run through a drying bin, and as the grain is dried, it is then transferred by augers to storage bins.
A typical drying bin forces warm dry air through a suitably apertured floor. The air then circulates around the grain particles, working its way up through the grain in the bin. In so doing, the air warms the grain and absorbs some of its moisture, and in turn the air is cooled and becomes moisture laden. In this manner, drying proceeds upwardly in zones through the drying bin. Periodically, as the grain is being dried, the warmest and driest layers from the bottom of the drying bin are drawn off, usually by means of sweep augers, and a discharge auger. A transfer auger transfer grain from the discharge auger to a storage bin.
Because the drying process can proceed at different rates, depending upon the moisture content of the grain, the type of grain, ambient air temperature and humidity and the intensity of the applied heat, it is necessary to provide some type of control system. Generally, it is convenient to allow the air heating and circulating equipment to operate according to its optimum design characteristics, and to control the overall drying by controlling the removal rate for the dried grain from the bottom of the bin. This in turn is done by controlling the sweep and discharge augers periodically according to a preset timer, intermittently according to sensed grain moisture, or by a combination of both.
The prior art has used many types of sensing systems for sensing, humidity or temperature of the grain or air at a selected zone. One type of system uses a sensing element placed at a point around the periphery of the drying bin at a preselected elevation above the floor. However, this type of system has certain inherent disadvantages because its operation depends on the assumption that uniform drying occurs at equal elevations above the floor. However, in practice there may be wet spots or zones which may be missed by this type of sensor. Other types of sensors are mounted at the discharge auger from the bin, for sampling the moisture content or temperature of the grain being discharged. In this type of system, the motor for the sweep and discharge augers is started periodically by a timer, then remains in motion until the temperature or wetness of the grain can be sampled at the discharge. If the grain is wet, the discharge mechanism is stopped to await another predetermined time interval while the grain drying apparatus continues in operation.
One difficulty that has been encountered with the type of system described above relates to the discharge of excessive amounts of wet grain in the process of discharging a large number of samples before the grain is fully dried. In a sample period at the end of a drying interval, the sweep and discharge augers must remain in operation for a sufficient interval of time, typically several minutes, to allow an accurate sample of grain from the bin to reach the moisture sensor location in the discharge tube, before a decision can be made as to stopping the augers for another drying interval, or keeping them going for unloading and transfer. During the initial stages of drying of a quantity of grain, a number of dry and sample cycles may be run through before the grain is ready for transfer, and the total amount of wet grain removed in the sample periods can present a problem.
Various types of moisture or temperature sensors have been used for sensing the condition of the grain at the discharge tube of a grain drying apparatus. One aspect of the present invention pertains to an improvement in a discharge grain temperature sensor. This sensor measures the temperature of the grain rather than measuring the moisture directly; however, since there is a direct correlation between moisture content and temperature of the grain exiting from the drying bin, the sensor can provide a convenient and useful measure of the grain moisture content.
When using a small sensing element such as a thermocouple, the dimensions of the sensing element are small compared with the dimensions of the discharge tube and the grain kernels. For this reason the contact between the grain and the thermocoupled on the inside wall of the discharge tube is relatively poor. This has the effect of decreasing the sensitivity and responsiveness of the control system to stop the discharge of grain when a zone of insufficiently dried grain has been reached.